Simulated moving bed absorption separation process

ABSTRACT

Disclosed is a method for the adsorptive separation by utilizing an improved simulated moving bed. This invention provides an improved formula for the calculation of primary flush flowrate by correspondingly associating the primary flush flowrate with each connection line by introducting a volume factor and carring out sequential control so as to reduce the amount of the flush stream and increase product purity and yield.

FIELD OF THE INVENTION

This invention relates to a method for adsorptively separating one ormore components through the selectivity of an adsorbent from a mixtureof feed containing both selectively adsorbed component A(containing oneor more components) and a relatively less adsorbed componentB(containing one or more components). More particularly, this inventionrelates to a method for separating a kind of isomer from a mixture ofhydrocarbon feed containing several isomers. This invention specificallyrelates to a method for producing high-purity para-xylene through theimproved adsorptive separation process from a mixture of feed containingpara-xylene and its other isomers.

BACKGROUND OF THE INVENTION

It is well known that adsorptive separation is one of the separationmethods widely adopted in the chemical industry, especially inpetrochemical industry. For quite some time, this method has beenadopted to separate a component which is difficult to be separated byother means from a mixture of feed containing various components.

In the prior art , there are abundant patent literatures describingmethods for separating one kind of hydrocarbon from the other isomers.For instance, methods for separating para-isomers of monocyclicaromatics substituted by dialkyl group from other isomers, especiallyfor separating paraxylene from other xylene isomers by employing aspecific zeolite molecular sieve adsorbent to preferably adsorbpara-isomers have been published in literatures of U.S. Pat. No.3,626,5020, U.S. Pat. No. 3,663,638, U.S. Pat. No. 3,665,046, U.S. Pat.No. 3,700,744, U.S. Pat. No. 3,686,342, U.S. Pat. No. 3,734,47, U.S.Pat. No. 3,394,109, U.S. Pat. No. 3,997,620, CN1022622, CN1022826,CN10493294, CN1051549A, CN1064071 and CN1047489A in which benzene,toluene, chlorobenzene, fluoro-aromatics, halogen toluene,para-dialkylbenzene,diethyltoluene, and tetraline are respectivelyrecommended as the desorbent depending on the composition of mixture ofthe feed.

A process of adsorptive separation may be effected both on a fixed bedor a moving bed , preferably on a simulated countercurrent moving bedsystem. For example, a simulated countercurrent moving bed system hasbeen adopted for adsorptive separation in U.S. Pat. No. 2,985,589, U.S.Pat. No. 3,268,604 and U.S. Pat. No. 3,268,605 while a rotary valve forthe system of simulated countercurrent moving bed has been disclosed inU.S. Pat. No. 3,040,777 and U.S. Pat. No. 3,422,848. Some drawbacks inthe prior art , the objects of this invention and the scheme ofsettlement will be further explained hereinafter with reference to theaccompanying drawing.

The drawing is a principle depiction of a continuous adsorptiveseparation system of a simulated countercurrent moving bed.

Referring to the drawing, a adsorptive separation system comprises fourzones, consecutively as adsorption zone, purification zone, desorptionzone and buffer zone. In the drawing, F represents a feed streamcontaining a selectively adsorbed component A and a relatively lessadsorbed component B, D represents a desorbent stream, E represents anextract, i.e. a stream of desorbent containing the selectively adsorbedcomponent A, R represents a raffinate, i.e. the remaining streamcontaining the relatively less adsorbed component B after desorption,H.sub.(in) and H.sub.(out) respectively represent an inflowing and anoutflowing stream enriched in said desorbent for the primary flushstream for the lines connecting the adsorptive beds, X represents thesecondary flush stream for the lines connecting the adsorptive beds, andM represents the simulated adsorbent moving direction shifted by therotary valve. Zone I is between F and R wherein the charged feedcontacts countercurrently with the adsorbent, and the selectivelyadsorbed component A shifts from the feed stream into the pores of saidadsorbent, displacing the desorbent D from the pores at the same time.Thus, zone I is defined as adsorption zone. Zone II is between F and E.For the reason that the adsorbent adsorbs selectively adsorbed componentA and a little amount of relatively less adsorbed component B as well,in zone II said adsorbent contacts with the stream containing only A andD coming from upstream of zone II, relatively less adsorbed component Bis displaced gradually from the pores by selectively adsorbed componentA and the desorbent D by means of appropriate adjustment of flowvelocity of the stream in the zone. As the adsorbent has a strongeradsorption selectivity to component A than to component B, component Awill not be completely displaced at the same time and thus will getpurified in the zone. Zone II is consequently defined as purificationzone. Zone III is between E and D wherein pure D contacts with theadsorbate purified in zone II and displace A from said adsorbent pores,thus this zone is defined as desorption zone. Zone IV is between D and Rwherein the flowrate of D is defined so that D is made to flow upwardsin the zone under flow control so as to prevent component B from gettinginto the stream in zone III to contaminate the extract. Thus, this zoneis defined as buffer zone .

In operation, switching equipment, e. g. rotary valve, etc. is employedto recycle the inflowing and outflowing streams and shift the four zonesin turn to realize simulation of adsorbent moving. During rotary valveswitching, it is necessary to flush the residue out of the adsorptivebed connection lines to ensure purity and recovery of the purifiedcomponent. Locations of H.sub.(in) , H.sub.(out) and X are shown in thedrawing and thus zones II and III are further divided into three morezones of II', II" and III'.

With regard to flowrate of H.sub.(in) and H.sub.(out), on one hand ifthe flowrate is set too small, the residue in the lines cannot beflushed away, which will subsequently affect the product purity andrecovcry; on the other hand, if the flowrate is too big, when the flushstream enriched in desorbent is drained through adsorptive beds afterflushing the connection lines, adsorptive space of the adsorbent will beoccupied by the desorbent in the flush stream , whereby weakening theadsorptive power of said adsorbent to the selectively adsorbedcomponent, which will also result in the decreased purity and recoveryof the selectively adsorbed component .

In the prior processes, the set flowrate W_(H)(in) and W_(H)(out) ofprimary flush stream to each adsorptive bed connection line iscalculated based on the following formula (I):

    W.sub.H(in) =W.sub.H(out) =2V.sub.L /T                     (I)

wherein:

V_(L) - - - volume of the longest connection line,m³

T - - - time interval of rotary valve switching, h

Based on volumetric equilibrium of the adsorber, the following formulascan be adopted for calculating flowrate in each zone;

    W.sub.I =W.sub.H +W.sub.S +W.sub.F

    W.sub.II' =W.sub.H +W.sub.S

    W.sub.II" =W.sub.S -W.sub.X

    W.sub.III =W.sub.S +W.sub.E -W.sub.X

    W.sub.III' =W.sub.S +W.sub.E +W.sub.H -W.sub.X

    W.sub.IV =W.sub.D +W.sub.H -W.sub.D -W.sub.X

wherein:

W_(H) - - - flowrate of the primary flush stream

W_(X) - - - flowrate of the secondary flush stream

W_(F) - - - flowrate of the feed stream

W_(E) - - - flowrate of the extract stream

W_(D) - - - flowrate of the desorbent stream

W_(S) - - - flowrate of the stream specified in zone II

In an actual system, however, because of the different position of eachbed spaced in the adsorber and different distance to the rotary valve,the volume of each line connecting bed with rotary valve varies a lot.For this reason, the calculation of the flowrate of primary flush streamfor each line based on formula (I) in prior proeess will result inhigher flush stream which will hence decrease the purity and recovery ofthe product.

SUMMARY OF THE INVENTION

The object of this invention is to provide an improved method ofadsorptive separation on simulated moving bed to increase recovery andpurity of product and reduce the flowrate of primary flush stream andproduction cost.

In the method according to the present invention, the flowrate of theprimary flush stream to each adsorptive bed connection line isassociated with the volume of each line to reduce the amount of theprimary flush stream so as to overcome drawbacks of higher flowrate offlush stream and lower purity and lower recovery of the product, whichare displayed by prior adsorptive separation process, and to realize theabovementioned object of this invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawing , this invention relates to a method toadsorptively separate a selectively adsorbed component from a relativelyless adsorbed compoonent in a mixture of feed containing both thecomponents by utilizing an improved adsorption system of simulatedcountercurrent moving bed. The method comprises the following steps:

Step 1) Adsorbing

A feed to be separated is made to contact in adsorption zone Icountercurrently with the adsorbent which selectively adsorbs componentA, leaving a raffinate R enriched in relatively less adsorbed componentB.

Step 2) Purifying

The adsorbent adsorbs component A and a small amount of component B aswell. This step is to let a portion of stream containing both componentA and the desorbent contact, in purification zone II, with the adsorbateafter step 1, displacing component B from the pores of said adsorbent,purifying component A.

Step 3) Desorbing

The desorbent contacts in desorption zone III with the adsorbateenriched in component A, desorbing the purified component A from saidadsorbent pores to obtain a extraet E. A portion of the extract is usedas the adsorbent for purification as mentioned hereinabove in step 2 andthe remaining is sent to distillation . High purity product of componentA is recovered and the desorbent is circulated for reuse.

Step 4) Buffering

In buffer zone IV, the stream of desorbent D is made to flow upwardsunder flowrate control, preventing the raffinate containing component Bfrom getting into the stream in the desorption zone to contaminate theextract.

Step 5) Switching

In the process of adsorptive separation, the four steps mentionedhereinabove are made to repeat in turn by using a stream switch-overdevice. Each zone is made to become adsorption zone , purification zone, desorption zone and buffer zone successively, realizing cyclic movingof the four zones in the system. Switching devices employed in thepreferred embodiment of this invention can be a rotary valve, aswitching valve, or any equipment with a function of stream shiftingRotary valves for the above functions recommended in the U.S. Pat. Nos.3,040,777 and 3,422,848 are incorporated herein by reference.

Step 6) Flushing

In the process of stream switch-over, it is necessary to flush the linesconnecting adsorptive beds with switching device to remove the residuein the lines prior to introducing new streams into each zone. A solutionenriched in desorbent is usually selected for flushing.

The flushing step is of utmost importance to the final recovery andpurity of the product. The flowrate 2V_(L) /T of the primary flushstream adopted by the prior process will not only result in excessiveflush stream and subsequently waste stream, but also have adsorptivespaces occupied by the desorbent in the flush stream, reducing the powerof the adsorbent to selectively adsorbed component, and reducing therecovery and purity of the selectively adsorbed component.

The inventors discover that if the flowrate of the primary flush streamsupplied to the lines connecting each bed is calculated and operated bythe following formula (II), the above mentioned shortcomings can beovercome to realize economical flushing, increased recovery and purityof the selectively adsorbed component, and higher capacity of the entireadsorptive separation system.

    W.sub.H(in)n W.sub.H(out)n =K.sub.n ·V.sub.L /T   (II)

wherein:

W_(Hn) - - - flowrate of the primary flush stream for flushing the lineconnecting the nth bed, m³ /h

K_(n) - - - volume factor of the nth connecting line, the value of whichcan be expressed by the following formula (III):

    K.sub.n =K'·V.sub.n /V.sub.L                      (III)

wherein:

K' - - - coefficient of volume factor, the value of which is in therange of from 0.5 to 3.5

V_(L) - - - volume of the longest conncetion line, m³

V_(n) - - - volume of the nth connection line, m³

T - - - time interval of rotary valve switching, h

In formula (II), the selection of K_(n) is related, on one hand, toV_(n) /V_(L) (different V_(n) /V_(L) for different line volume),on theother hand, to the chosen coefficient K' of volume factor. The value ofcoefficient K' depends on the length, diameter and shape of a connectionline. For lines having shorter length, bigger diameter and lessbendings, smaller K' (0.5-2) is appropriate, e.g. 1˜1.5. For lineshaving longer length, smaller diameter and more bendings, biggerK'(2˜3.5) is suitable, e.g. 2.5˜3.0.For lines of intermediate extent,coefficient K' of 2 is preferable.

By chosing an appropriate coefficient of the volume factor, the flowrateof the primary flushing stream can be set in the most reasonable way byhaving it associated with the volume of each connection line andadjusted with sequential control to assign different flushing flowratesto different connection lines, which can not only save flushing streambut also increase recovery and purity of the selectively adsorbedcomponent.

By using the improved method of this invention and under theprerequisite of ensuring flushing effect, the flowrate of the primaryflush stream can be set smaller so as to reduce the space originallyoccupied by the desorbent, getting more adsorptive space for desiredproduct. In this way, the recovery can be increased by 4˜5%, the purityby 0.1˜0.2%,and the unit capacity indirectly by 3˜5%.

The simulated moving bed system disclosed in this invention can compriseone or more adsorbers, each of which may contain any reasonablequantities of adsorptive beds , usually eight or more beds, e.g. 8˜24beds. Beds of integral multiple of eight are preferable, e.g. 24 beds.

The feed applicable for the simulated moving bed adsorption system ofthis invention can be any feed suitable for adsorptive separation. Thefeed can be a mixture of various components having different adsorptiveproperties produced in petrochemical process. Said mixture may includevarious paraffins and substituted paraffins, e.g. haloalkanes,cycloalkanes, olefines, etc., various aromatics and substitutedaromatics, e.g. alkyl aromatics, halogenated aromatics, heterocyclicaromatics, etc., and alkyl amines, alkanols, alkyl ethers, alkyl esters,etc.

In the preferred embodiments of this invention, the feed to be separatedmay be various applicable paraffins or a mixture of C₈ and C₉ aromatics.In the specially preferred embodiment ,the feed is a mixture of variousisomers of dialkyl benzene, especially xylene.

The adsorbent adopted in the simulated moving bed adsorption system usedin this invention can be any applicable adsorbent that can separate thefeed stream. Selection of adsorbent is primarily based on the propertiesand concentrations of various components in the feed, and theinteraction of components with said adsorbent. Different natural orsynthesized adsorbents can be chosen for different feed compositions,such as natural or synthesized zeolites, e.g. molecular sieves of X or Ytype, activated alumina and silica gel, and moleculatr sieve adsorbentsof aluminosilicate with different metallic cations by ion exchangedeveloped since the last ten years.

The desorbent adopted in the present adsorptive separation system usedin this invention can be any liquid which has a different adsorptivepower as compared to already adsorbed component, and can displace theselectively adsorbed component from said adsorbent which can becontinuously used in operation. The desorbent to be selected should becompatible with various components in the feed and the adsorbent, andcan be used repeately after recovery by easy separation from thosecomponents by other means, e.g. distillation. The desorbent adopted inthe present process includes various applicable paraffins, aromatics andtheir substituents.

Conditions for adsorptive separation comprise temperature ranging fromambient temperature to about 250° C.,with the range between 60° C. ˜200°C. being preferred, and pressure ranging between 140 atmosphericpressures. Conditions for desorption are the same as those foradsorption.

The following examples are for the purpose of further demonstration onthe effect of the adsorptive separation with decreased amount of primaryflush stream involved in the preferred embodiment of this invention.However, the method in this invention is not limited to the examplesillustrated herein. Exactly in reverse, the scope of this inventionincludes all methods of adsorptive separation with reduced amount ofprimary flush stream.

EXAMPLE 1

This example illustrates separation of feed containing para-xylene andits isomers using the following process conditions:

Composition of feed: p-xylene,o-xylene,m-xylene, ethylbenzene ,etc.

Adsorbent: X type zeolite molecular sieve containing potassium andbarium

Desorbent: paradiethylbenzene

Primary flush stream H(in): solution enriched in paradiethyl- benzene

Temperature: 177° C.

Pressure: 0.88 MPa.

Flowrates of the feed stream through the adsorber (loaded 95%):

    W.sub.F =224m.sup.3 /h

    W.sub.D =336m.sup.3 /h

    W.sub.E =132m.sup.3 /h

    W.sub.X =18m.sup.3 /h

    W.sub.K =446m.sup.3 /h

Time interval of rotary valve switching was:

    T=101.4s=0.0281 h

The adsorber contained 24 beds. Volumes of lines connecting each bed areshown in Table 1.

Selected value of the coefficient K' of volume factor was 2, volumefactors K_(n) of each bed connecting line were calculated according to2V_(n) /V_(L),which are also shown in Table 1.

Based on the value T, volume V_(L) of the longest bed connection line,and volume factors K_(n) of each bed connection line, the flowratesW_(H)(in)n and W_(H)(out)n of the primary flush stream can be calculatedby the formula:

    W.sub.H(in)n W.sub.H(out)n =K.sub.n V.sub.L /T,

which are also shown in Table

                  TABLE 1    ______________________________________    Values V.sub.n, K.sub.n and W.sub.H(in)n of bed connection    ______________________________________    lines    V.sub.n          0.74   0.59    0.55 0.54  0.57 0.56 0.67  0.69    K.sub.N          1.50   1.20    1.10 1.10  1.20 1.10 1.40  1.40    W.sub.H(in)n          53.4   42.7    39.1 39.1  42.7 39.1 49.8  49.8    V.sub.n          0.68   0.68    0.71 0.77  1.00 0.85 0.70  0.71    K.sub.N          1.40   1.40    1.40 1.50  2.00 1.70 1.50  1.40    W.sub.H(in)n          49.8   49.8    49.8 53.4  71   60.5 53.4  49.8    V.sub.n          0.70   0.64    0.68 0.64  0.60 0.59 0.59  0.63    K.sub.N          1.40   1.50    1.40 1.30  1.20 1.20 1.20  1.30    W.sub.H(in)n          49.8   53.4    49.8.                              46.3  42.7 42.7 42.7  46.3    ______________________________________     *V.sub.L : volume of the longest connection line.

By associating W_(H)(in) and W_(H)(out)n shown in Table 1 with each linevolume of the 24 bed connection lines respectively, adjustment on theflowrate of primary flush stream to each line can be realized bysequential control. When operation got stable, the recovery of p-xylenereached up to 95%, and the product purity 99.46%.

Comparative Example 1

Process conditions for comparative example 1 were as same as those forexample 1, except for the values of W_(H)(in) and W_(H)(out) which werecalculated based on the following formula:

    W.sub.H(in) =W.sub.H(out) =2V.sub.L /T=2×1.00/0.0281=71(m.sup.3 /h)

The recovery for p-xylene was 90%, preduct purity 99.3%.

By contrasting the primary flush flowrate for each line in example 1with the flush rate 71m³ /h for each line in comparative example 1, itis apparent that the amount of primary flush stream employed in thepreferred embodiment of this invention is 15˜45% less than that in theprior process, average 30% less or more. As the amount of flush streamin comparative example 1 is more than that in example 1, it willinevitably result in lower recovery of the selectively adsorbedcomponent and lower product purity as well.

EXAMPLE 2

The process in Example 2 was carried out under the following parameterswith all other conditions being as same as in example 1.

    W.sub.F =235 m.sup.3 /h

    W.sub.D =355 m.sup.3 /h

    W.sub.E =140 m.sup.3 /h

    W.sub.X =19 m3 h

    W.sub.K =467 m.sup.3 /h

Time interval of rotary valve switching was:

    T=99.3 s=0.0275 h

Selected value of the coefficient K' of volume factor was 1.8, volumefactors K_(n) of each line were calculated by 1.8V_(n) /V_(L), which areshown in Table 2.

Based on the value T, volume V_(L) of the longest bed connection line,and volume factors K_(n) of each bed connection line, the flowratesW_(H)(in)n and W_(H)(out)n of the primary flush stream can be calculatedby the formula

    W.sub.H(in)n =W.sub.H(out)n =K.sub.n ·V.sub.L /T,

which are also shown in Table 2.

                  TABLE 2    ______________________________________    Another group of values K.sub.n and W.sub.H(in)n of bed connection    ______________________________________    lines    V.sub.n          0.74   0.59    0.55 0.54  0.57 0.56 0.67  0.69    K.sub.n          1.33   1.06    0.99 0.97  1.03 1.01 1.21  1.24    W.sub.H(in)n          48.4   38.5    36.0 35.3  37.5 36.7 44.0  45.1    V.sub.n          0.68   0.68    0.71 0.77  1.00 0.85 0.70  0.71    K.sub.n          1.22   1.22    1.28 1.39  1.80 1.53 1.26  1.28    W.sub.H(in)n          44.4   44.4    46.5 50.5  65.4 55.6 45.8  46.5    V.sub.n          0.70   0.64    0.68 0.64  0.60 0.59 0.59  0.63    K.sub.n          1.26   1.15    1.22 1.15  1.08 1.06 1.06  1.13    W.sub.H(in)n          45.8   41.8    44.4 41.8  39.3 38.5 38.5  41.1    ______________________________________

By associating W_(H)(in)n and W_(H)(out)n shown in Table 2 with eachline volume of the 24 lines respectively, adjustment on the flow ofprimary flush stream to each line can be realized by sequential control.When operation got stable, the recovery of p-xylene reached up to 96%,and the product purity 99.5%.

By contrasting example 2 with example 1, it is apparent that therecovery and purity of p-xyline have been further increased by furtherreducing the flow rates of primary flush stream.

Comparative Example 2

Process conditions for comparative example 2 were as same as those forexample 2, except for that the primary flush stream flowrate W_(H)(in)nand W_(H)(out)n for all lines was 65.4m³ /h, which are calculated basedon formula (II). When operation got stable, the recovery of p-xylene was90.2%, and the product purity 99.3%.

By contrasting the flowrates of primary flush stream for each bedconnection line in example 2 with the flush rate of 65.4m³ /h incomparative example 2, it is apparent that the amount of primary flushstream in the preferred embodiment of this invention is 15·46% less thanthat in the prior process, average 32% less or more.

EXAMPLE 3

This example illustrates the adsorptive separation process for a mixtureof feed containing m-xylene and its isomers with the following processconditions:

Composition of feed: p-xylene,o-xylene,m-xylene, ethylbenzene ,etc.

Adsorbent: zeolite molecular sieve

Desorbent: toluene

Primary flush stream H(in): solution enriched in toluene

Temperature: 180° C.

Pressure: 0.90 MPa.

The adsorber contains 24 beds, and time interval of rotary valveswitching was:

    T=90s=0.025 h

Selected value of coefficient K' of volume factor was 1.90, on which theflowrates of primary flush stream to each line were calculated.Sequential control was utilized to adjust the flowrates of primary flushstream to each line. When operation got stable, the recovery form-xylene reaches up to 60%,and the product purity 98%.

Comparative Example 3

Process conditions for comparative example 3 were as same as those forexample 3, except for that W_(H)(in), and W_(H)(out) were calculated bythe formula:

    W.sub.H(in) =W.sub.H(out) =2V.sub.L /T

The recovery for m-xylene was 51%, and the product purity 95.0%.

By contrasting example 3 with comparative example 3, we can notice thatdecreased amount of primary flush stream demonstrated in the preferredembodiment of this invention may greatly increase the recovery andpurity of the product.

Although only a few examples and their comparisons are illustrated inthis invention, this invention is not limited to the illustrationsherein to those skilled in the art. Just the opposite , many changes andmodifications could be made on this basis . The scope of themodification and changes will be defined in the following claims.

What is claimed is:
 1. A method for adsorptively separating aselectively adsorbable component from a relatively less adsorbablecomponent in a feed mixture using a simulated countercurrent moving bedadsorption system comprising a plurality of absorption beds, the methodcomprising the steps of:(1) supplying a feed mixture containing aselectively adsorbable component A and a relatively less adsorbablecomponent B to an adsorption zone I, wherein the feed mixture contactscountercurrently with an adsorbent, said adsorbent selectively adsorbingfrom the feed, component A and to a lesser extent component P in poresof the adsorbent, thereby forming an adsorbate and a raffinate, theraffinate being enriched in component B; (2) contacting a streamcontaining component A and a desorbent with the adsorbate from step (1)in a purification zone II to displace component B from the pores of theadsorbent, thereby purifying component A in the pores of the adsorbentto provide a purified adsorbate; (3) contacting a desorbent with thepurified adsorbate from step (2) in a desorption zone III to desorbcomponent A from the pores of the adsorbent, thereby providing anextract enriched in component A, a first portion of the extract beingused as the stream in step (2) and distilling the remainder of theextract to provide a high purity product A; (4) controlling the flowrateof a desorbent stream in a buffer zone IV to prevent the raffinate fromentering the desorption zone III and contaminating the extract; (5)cyclically rotating zones I, II, III and IV by employing a rotary valvestream switching device to recycle inflowing and outflowing streams and,during the switching, using a flush stream enriched in desorbent toflush out residue in lines connecting the adsorption beds, a primaryflush stream flow rate W_(h) for each bed being calculated according tothe formula:

    W.sub.H(in)n =W.sub.H(out)n =K.sub.n ·V.sub.L T

wherein: K_(n) is the volume factor of the line connecting the nthadsorption bed, the value of the factor being expressed by the formula:

    K.sub.n =K'·V.sub.n /V.sub.L

wherein: V_(n) represents the volume of the nth connection line,measured in m³ ; V_(L) represents the volume of the longest connectionline, measured in m³ ; T represents the time interval of rotary valvestream switching, measured in hours; and K' represents the coefficientof volume factor of each connection line, which factor is from 0.5 to3.5,wherein the flush stream flowrate for each flush stream issequentially controlled in such a manner that a respective flush streamflowrate is adopted for each connection line.
 2. The method according toclaim 1, wherein the value of K_(n) is 2 ·V_(n) /V_(L).
 3. The methodaccording to claim 1, wherein the feed mixture comprises a mixture ofparaffins and isomers of substituted paraffins.
 4. The method accordingto claim 1, wherein the feed mixture comprises a mixture of aromaticsand isomers of substituted aromatics.
 5. The method according to claim4, wherein the aromatics comprise dialkyl benzene, dialkylphenol,dinitrobenzene, dihalogeno-benzene and dialkyl naphthalene.
 6. Themethod according to claim 5, wherein the dialkyl benzene is para-xylene.7. The method according to claim 6, further comprising recovering thepara-xylene as a product.
 8. The method according to claim 1, whereinthe adsorbent is an X type zeolite, a Y type zeolite, or a crystallinealuminosilicate with a metal cation introduced by ion exchange.
 9. Themethod according to claim 1, wherein the desorbent is a paraffin or asubstituted paraffin, or an aromatic or a substituted aromatic, thedesorbent being effective to displace the selectively adsorbedcomponent, and being compatible with the adsorbent and the feed stream,and being readily separable from component A.
 10. The method accordingto claim 1, wherein the adsorption system comprises one or moreadsorbers.
 11. The method according to claim 1, wherein the adsorptionsystem comprises eight or more adsorption beds.
 12. The method accordingto claim 11, wherein the number of the adsorption beds is an integralmultiple of eight.
 13. The method according to claim 1, wherein theadsorbent is a K--Ba--X X-type molecular sieve and the desorbent isparadiethylbenzene.